The present invention relates to a magnetic bubble memory device and, more particularly, to a magnetic bubble memory device using both soft magnetic element tracks and ion-implanted tracks in which a bias margin which is common to both tracks is made wide.
The magnetic bubble memory device which has been conventionally practically used is of the type called a permalloy device and its fundamental concept is that a cylindrical magnetic domain, i.e., a bubble 2 which exists on a magnetic garnet film, e.g., on a (YSmLuCa).sub.3 (FeGe).sub.5 O.sub.12 garnet film is transferred along a permalloy track 1 provided on the garnet film as shown in FIG. 1 due to the rotating magnetic field applied in the surface. A bubble generator, a transfer gate, a swap gate, a replicator, etc. consist of the permalloy pattern 1 and a conductor pattern 5 provided between a garnet film 3 and the permalloy pattern 1 through insulating layers 4 and 6 as shown in FIG. 2. Each function is performed by allowing a control pulse current to flow through the conductor pattern 5. (In FIG. 1, a non-magnetic substrate is not shown.)
For a high density and high integration bubble memory device, a pattern width of the permalloy track 1 shown in FIG. 1 and the dimension of the gap between adjacent permalloy elements have been remarkably made fine. For example, when a device of a bit period of 8 .mu.m is formed and bubbles therein have a diameter of about 2 .mu.m, it is necessary to set the pattern and gap dimensions to be as fine as about 1 .mu.m. In the future, to realize a further high density for a device by means of a permalloy device, it is necessary to form a fine permalloy pattern of 1 .mu.m or less over the whole chip and this method is technically very difficult.
On the contrary, recently, a bubble memory device of a new type is being highlighted. This new device is characterized in that the track is formed by implanting ions in place of a conventional permalloy track and is called an ion implanted device. Namely, as shown in FIG. 3, a mask (not shown) having a shape of a contiguous disk (a bead-like pattern) is formed on the garnet film 3 and ions such as Ne or H.sub.2 ions or the like are implanted into the garnet film surface to form an ion implanted region 7 in the portion outside the contiguous disk, and the direction of the magnetization of region 7 is made parallel with the surface of the garnet film 3. In this way, by applying a rotating magnetic field to the surface in parallel thereto, a bubble is transferred along the edge of a contiguous disk 8 (a propagation track) in a similar manner to the permalloy device. A feature of this ion implanted device is that the pattern dimension of the track 8 may be as large as approximately twice that of the track of the permalloy device and since its manufacture is easy, it is fitted to a device of a high density.
However, this ion implanted device has such drawbacks that the operations of the replicator, transfer gate, swap gate, etc. are not sufficiently stable and that there is no block replicator; therefore, this causes a problem when such a device is made fit for practical use.
Therefore, as a magnetic bubble memory device having a replicator, transfer gate, swap gate, etc. which sufficiently stably operate and having bubble tracks of a high density, a magnetic bubble memory device has been proposed in which tracks formed by means of ion implantation and tracks formed by permalloy are both used, thereby making the most of the above-mentioned features of the permalloy and ion-implanted devices. That is, the tracks formed by implanting ions are used as minor loop tracks, while at least a part of the major loop (or line) is formed by permalloy.
A bubble memory device is generally constituted by a minor loop group 9 serving as an information memory section and a major line or major loop serving to write and read information, as shown in FIG. 4. A transfer gate, swap gate, replicator, and the like 11 are used for junctions between the minor loops 9 and the major lines (or major loops). The minor loops 9 are constituted by the bubble propagation tracks and occupy the greater part of the device area. The major lines or loops (hereinafter referred to as merely "major lines" for simplicity sake) comprise: bubble tracks 10 and 12; a bubble generator 13; and a bubble detector 14. The minor loops 9 and the major lines are connected through the transfer gate, swap gate, replicator, and the like. In a device of such a type, the minor loops 9 occupying the greater part of the device area are formed by the ion-implanted tracks which are appropriate for realizing a high density and at least a part of the replicator and/or transfer gate in the major lines is formed by permalloy. The major lines, that is, the replicator, etc. occupy only a small part of the device area. Therefore, formation of these major lines by permalloy is possible, although the pattern dimension is more strictly limited than the ion-implanted tracks in the minor loop group. In addition, it is also possible to constitute the major lines so that the bit period thereof is, for example, two to four times that of the minor loop group. Such an arrangement alleviating the restriction in pattern dimension of the permalloy make the manufacture of the bubble memory device easy.
However, the magnetic bubble memory device which adopts the two kinds of bubble propagation tracks as described above has a problem such that there is a disagreement in bias magnetic field margin between the ion-implanted tracks and the soft magnetic element tracks.
FIG. 5 is a schematic diagram of a cross sectional structure of a magnetic bubble memory device using both soft magnetic element tracks and ion-implanted tracks (hereinafter, simply referred to as a hybrid device). In this structure, the ion-implanted regions 7 are formed in a part of the magnetic garnet film 3 and the soft magnetic element track 1 is formed on other regions through a spacer 15 therebetween.
In a hybrid device with such a structure as shown in FIG. 6, there occurs such a disagreement in the bias magnetic field margin that the bias magnetic field margin 16 of the soft magnetic element tracks is higher than the bias magnetic field margin 17 of the ion-implanted tracks, which causes a problem such that the common bias magnetic field margin is decreased.